Micromachined silicon structures frequently are used to detect and measure acceleration through the use of a differential capacitor. In such sensors, a movable mass is positioned between two plates so that one capacitor is formed by one plate and the mass and a second capacitor is formed by the second plate and the mass.
The application of a force along a sensitive axis causes the mass to move relative to the plates, causing a change in the capacitances in the two capacitors of the differential capacitor, which causes a signal to appear on the mass that reflects the amount of acceleration. An accelerometer based on this principle and a process for fabricating such an accelerometer are described in commonly assigned U.S. Pat. Nos. 5,345,824 and 5,314,572.
Positive and negative bias voltages are applied, respectively, to the two plates, to provide an electrostatic force for positioning the mass. With a negative feedback loop, a voltage may be applied to the mass to create an electrostatic force that will offset the acceleration and maintain the mass centered between the plates.
Although this structure is well-suited to measure large accelerations (on the order of 50 g's), it is inadequate to measure accelerations on the order of 5 g's, where greater sensitivity is required. As described in commonly assigned U.S. patent application Ser. No. 08/347,795, entitled Sensor With Separate Actuator and Sense Fingers, and filed Dec. 1, 1994 by Roger Howe and Stephen Bart, which is incorporated herein by reference, this greater sensitivity is achieved by separating the sensing and force-feedback (actuator) functions.
With the separated sensing and force-feedback functions, ideally there should be no electrostatic force between the movable mass and the fixed sense capacitor plates. However, any signal that is present on the movable mass may create a net electric force between the mass and the two fixed sense capacitor plates, which distorts the accuracy and sensitivity of the accelerometer.
The present invention overcomes these problems as will be shown in the remainder of the specification referring to the attached drawings.